How to Fix a Slow Computer

I restart the computer and press F8 so it goes into Safe Mode. That way it only runs Windows XP without background programs. I go to Program Files>Accessories>System Tools>Disk Cleaner or LeftClick on My Computer then RightClick on C: drive and choose Properties to run DiskCleaner with Compress Drive unchecked. The HardDrive needs about 10 to 15% space to do a Defrag. (Freespace / Capacity = %). After the HardDrive's been cleaned I click on the Properties Tool tab for Disk Defragmenter, or Program Files>Accessories>System Tools>Disk Defragmenter. Let it defrag while no other programs are running. After it finishes, RightClick on C: drive and choose Properties, click on Tools tab, click on Error Checking "Check Now" and place checks in both checkboxes. Restart computer, and let error checking do its job.
If it still runs slow I suggest getting the free Malwarebytes Anti-Malware installed and do a Full System Scan, then run it again after Restart. Then download and install IOBit's Advanced System Care Free Edition. CCleaner Slim finds a few more items after that.
Microsoft Malicious Software Removal should be updated every month, along with Windows Update items.
Buy an external USB HardDrive and move your photos, music and anything else you want to keep onto it. The less files there are on the HardDrive the easier it is on your computer.
I've seen a few laptops here for my friends stationed here in Afghanistan where they were so infected the browser window was blocked. I've used this remedy and other methods to get them back online.